Known picture-in-picture television receiver include two video signal channels, a main and an auxiliary channel, each including a tuner; an IF chain; and a video detector. The information from the auxiliary channel is compressed and stored in a memory in synchronism with the auxiliary video signal. This stored information is retrieved in synchronism with the main video signal and replaces a portion of the main video signal at a predetermined image location. In this way, a picture-in-picture video signal is formed representing an image having a first region which displays the main video image, as represented by the main video signal and a second, usually smaller, region which displays the auxiliary video image, as represented by the auxiliary video signal.
An NTSC standard video signal consists of successive frames of 525 lines made up of two interlaced fields of 2621/2 lines each. Odd fields, which contain lines 1, 3, 5, . . . etc., alternate in time with even fields, which contain lines 2, 4, 6, . . . etc. to form the 525 line image. A sampled data processed NTSC signal sample at a rate of, for example, four times the color subcarrier frequency contains 910 samples in each line.
The auxiliary video signal is compressed by, for example, storing in the memory only every third sample of every third line. Each field of compressed auxiliary image information, thus, comprises 87 lines of 303 samples each. Odd compressed fields may contain information from every third odd line, e.g. lines 1, 7, 13, . . . etc., and even compressed fields may contain information from every third even line, e.g. lines 4, 10, 16, . . . etc., of the 525 line image.
In every NTSC field of video information, 21 lines form a vertical blanking interval (VBI) and do not contain image information. These lines need not be compressed, stored or displayed in the inserted auxiliary image. Only the remaining 242 lines contain image information and need be compressed into 80 lines (1/3 of 242), stored and displayed to form the auxiliary image. In addition, approximately 150 samples of each line of sampled data video information, form a horizontal blanking interval (HBI) and do not contain image information. These samples need not be compressed, stored in memory or displayed in the inserted auxiliary image. Only the remaining 760 samples contain image information and need be compressed into 253 samples (1/3 of 760), stored in memory, and displayed to form the auxiliary image.
In each field of the main video signal, a portion, consisting of 253 adjoining samples of 80 adjoining lines, is replaced by the previously stored field of compressed auxiliary samples. If this portion is located in the lower right hand corner, for example, samples 607 through 859 (totaling 253 samples) of lines 182 through 261 (totaling 80 lines) of each field of the main video signal are replaced with the previously stored compressed auxiliary video samples to form the picture-in-picture video signal. In an odd field of the main video signal, the affected lines are lines 363, 365, 367, . . . 519 and 521 (totaling 80 lines) of the 525 line picture-in-picture video signal image. In an even field, the affected lines are lines 364, 366, 368, . . . 520 and 522 (totaling 80 lines) of the 525 line picture-in-picture video signal image.
The system described above inserts previously stored samples representing the auxiliary video signal into a sampled data main video signal. Alternatively, the previously stored samples may be converted into a continuous signal and inserted into a corresponding portion of the continuous main video signal.
If the previously stored field of the auxiliary video signal is from an odd field, and it is inserted into an odd field of the main video signal, and an even field of the auxiliary video signal is inserted into an even field of the main video signal, then lines 363, 364, 365, 366, etc. of the 525 line picture-in-picture video signal contains lines 1, 4, 7, 10, etc. of the 525 line auxiliary video signal respectively.
If, however, the previously stored field of the auxiliary video signal is from an even field and it is inserted into an odd field of the main video signal, and an odd field of the auxiliary video signal is inserted into an even field of the main video signal, then lines 363, 364, 365, 366, etc. of the 525 line picture-in-picture video signal contains lines 4, 1, 10, 7, etc. of the 525 line auxiliary video signal, respectively. The interlace of the auxiliary video image in the picture-in-picture video image is, thus, inverted and the display is objectionable. This situation, when detected, must be corrected so that proper interlacing may be maintained. The interlace inversion condition may be detected by comparing the timing of a main odd/even signal to an auxiliary odd/even signal.
The interlace inversion condition may be corrected by rearranging the lines of the auxiliary video signal in the picture-in-picture video image. When the interlace inversion condition is detected, the inserted picture is constructed as follows. The lines inserted into the odd field of the main video signal are retrieved and inserted in the normal manner. The order of the lines inserted into the even field, however, is modified in that the first line inserted into the main video signal is the second line previously stored instead of the first. That is, when the interlace inversion condition is detected and when an even field of the main video signal is being scanned, line 1 of the auxiliary video signal is not displayed as the top line of the inserted image. Instead line 7 of the auxiliary video signal is displayed as the top line of the inserted image. As a result, lines 349, 350, 351, 352, etc. of the 525 line picture-in-picture video signal include lines 4, 7, 10, 13, etc. respectively, of the auxiliary video image. This sequence is properly interlaced.
Dual port memories have recently become available which have a high memory capacity, that is they are capable of storing a full field of video information, and are priced such that integration into a consumer television receiver is economically practical. The HM 53051 P, 262,144 word 4-bit frame memory, manufactured by Hitachi, is such a dual port video memory system. These high capacity memory chips allow flexibility in use which was previously unavailable with lower capacity memory integrated circuits.
Such a memory may be envisioned as being subdivided into three blocks, each capable of storing one field of compressed auxiliary video information. Successive fields of compressed auxiliary video information are written into these blocks in round robin fashion. Fields of previously stored compressed auxiliary video information are retrieved from the blocks, also in round robin fashion, so that no block is written into and read from simultaneously.
The HM 53031 P memory operates differently from normal random access memories (RAMs). Normal RAMs include a data input terminal and an address input terminal. Both a data sample and an address corresponding to the storage location into which the sample is to be stored must be supplied for each sample to be stored. The HM 53031 P also includes a data input terminal and a write address terminal. Only one initial write address is applied to the write address terminal. The memory generates subsequent successive addresses internally to store samples in sequential storage locations beginning at the location corresponding to the initial write address. The HM 53031 P further includes a data output terminal and a read address terminal. Retrieval of data operates in a similar manner. An initial read address is applied to the read address terminal. Samples are retrieved from sequential locations in the memory beginning at the location corresponding to the initial read address. In the remainder of this application, such a memory will be referred to as a self-sequencing memory. It is desirable to use such a self-sequencing memory in a picture-in-picture video signal generator.